Picture the scene:
You get a song stuck in your head - in my case the current one is 'Orange Coloured Sky', not sure why - anyway, said song keeps going around and around until it starts to irritate you.
You spend the best part of quarter of an hour trying to get rid of said song by replacing it with something else, anything come to that until you finally get rid of it...
Then about half an hour to an hour later you find yourself humming the first song again! And you will probably find that you didn't even notice when it reappeared in your head. [A bit like taking pain killers - you don't really notice exactly when they take effect but you sure do notice when they wear off!]